The present invention relates to a semiconductor laser, a optical module using the same, and a optical communication system. More particularly, the invention relates to a semiconductor laser having, on a substrate crystal, an active layer which emits light and a cavity structure for obtaining a laser beam from the light generated from the active layer, in which a regrown layer is formed near the active layer, a optical module having the semiconductor laser as a component, and a optical communication system using the semiconductor laser or optical module.
The speed of information transfer of recent years is requested to be rapidly increased. Consequently, a optical communication of transfer speed of 10 Gb/s or higher is being developed. For a optical communication, usually, a optical module in which a semiconductor laser, a detector, driving circuits for the semiconductor laser, and the like are assembled is used.
As a optical communication system using the optical module and whose transfer speed exceeds 10 Gb/s, a system as shown in FIG. 9 is known. A optical module 907 transmits signal light from a semiconductor laser 901 in accordance with an external circuit 908 which operates the optical module 907. A optical signal transmitted from a optical module on the other side is received by a detector 905 driven by a detector driving circuit 906. All of optical signals are transferred at high speed via optical fibers 909.
As the semiconductor laser 901, an edge emitting laser in which a gallium indium phosphide arsenide (GaInPAs) semiconductor material is used for an active layer is mainly used. Generally, a GaInPAs laser has a drawback such that when device temperature increases, a threshold current largely increases. It is therefore necessary to assemble a thermoelectronic device 904 for stabilizing temperature and an automatic power control (APC) circuit for always measuring fluctuations in a optical output from the semiconductor laser 901 by a detector 903 for monitoring and feeding back the measurement value to a laser driving circuit 902.
Consequently, the number of parts constructing the optical module 907 is large, the driving circuit is complicated and has a large size and, accordingly, the cost of the optical module itself is high.
On the other hand, attention is being paid to a vertical cavity surface emitting laser (VCSEL) as a light source suitable for a high-speed optical module. The vertical cavity surface emitting laser is constructed by an active layer for generating light and an optical resonator taking the form of a pair of reflecting mirrors disposed so as to sandwich a current blocking layer for injecting current to a very small region in the active layer and the active layer. The cavity length of the vertical cavity surface emitting laser is only a few xcexcm which is much shorter than the cavity length (few hundreds xcexcm) of an edge emitting laser, and the volume of the active region is small, so that the vertical cavity surface emitting laser has an excellent high-speed characteristic. Further, the vertical cavity surface emitting laser has excellent advantages such that due to an almost circular beam shape, the beam is easily coupled to a optical fiber, a cleavage process is unnecessary, a device inspection on a wafer unit basis can be made, laser oscillation is carried out with a low-threshold current, the power consumption is low, and the cost is also low.
With respect to laser oscillation wavelength, in recent years, oscillation of a vertical cavity surface emitting laser of a 1.3 xcexcm band made of a new semiconductor material which can be formed on a substrate made of gallium arsenide (GaAs) such as gallium indium nitride arsenide (GaInNAs) or gallium arsenide antimonide (GaAsSb) has been reported one after another. Expectations are running high for practical use of a longer wavelength region vertical cavity surface emitting laser adapted to a single mode fiber capable of performing long-distance high-speed transfer. Particularly, in the case of using GaInNAs for an active layer, electrons can be confined in a deep potential well in a conduction band, and it is expected that the stability of characteristics with respect to temperature can be largely improved.
By the above advantages, if the longer wavelength region vertical cavity surface emitting laser is realized, it is expected that a higher-performance and lower-cost optical module suitable for use in a LAN can be achieved.
The length of the optical resonator of the vertical cavity surface emitting laser is remarkably short. To generate laser oscillation, it is necessary to set the reflectance of upper and lower reflecting mirrors to an extremely high value (99.5% or higher). As a reflecting mirror, a multilayer reflecting mirror obtained by alternately stacking two kinds of semiconductors of different refractive indices having a thickness of a quarter of the wavelength (xcex/4 n: xcex denotes a wavelength, and n denotes a refractive index of a semiconductor material) is mainly used.
To obtain high reflectance by the smaller number of layers stacked, it is desired that the difference between the refractive indices of the two kinds of semiconductor materials used for the multi-layer reflecting mirror is as large as possible. In the case where the material is semiconductor crystal, to suppress misfit dislocation, it is preferred that the semiconductor crystal is lattice-matched with the substrate material. Under present conditions, a multilayer reflecting mirror made of a GaAs/aluminum arsenide (AlAs) semiconductor material or a dielectric material such as silicon dioxide (SiO2) or titanium dioxide (TiO2) is mainly used. The current blocking layer is indispensable to lower the threshold current, disposed between the active layer and an electrode for passing a current, and plays the role of limiting the current injected to the active layer to a very small region (hereinbelow, described as an aperture). To realize a single lateral mode, the diameter of the aperture has to be small as 2 to 3 xcexcm at an oscillation wavelength of 850 nm or 5 to 6 xcexcm at an oscillation wavelength of 1300 nm. Concretely, a method of selectively oxidizing an AlAs layer introduced into a device structure in the lateral direction to change the layer to an aluminum oxide (AlxOy) insulating layer, thereby blocking current only by a very small AlAs region remained in the center is in the mainstream at present. There is also a method of blocking current by burying either a semiconductor material having a large band gap or a material doped with an impurity of a conduction type opposite to the conduction type of the device.
On the other hand, in order to realize a optical module having a high speed characteristic over 10 Gb/s, a vertical cavity surface emitting laser used as a light source has to achieve a high speed characteristic over 10 Gb/s. Consequently, it is indispensable to reduce resistance (R) and capacity (C) of the vertical cavity surface emitting laser. FIG. 5 shows the relations of resistance, capacity, and modulation characteristic. The capacity of a vertical cavity surface emitting laser is generally a few hundreds fF. To achieve high-speed modulation over 10 Gb/s, the device resistance has to be reduced to at least 50 xcexa9 or lower.
For a vertical cavity surface emitting laser, as described above, a multi-layer reflecting mirror made of an AlAs/GaAs semiconductor is mainly used. In a conventional device, an electrode is disposed on an upper p-type AlAs/GaAs semiconductor multilayer reflecting mirror and a current is injected to an active layer via the semiconductor multilayer reflecting mirror. At this time, there is a problem that the energy difference of a valence band of the AlAs/GaAs semiconductor becomes a large resistance component in a heterojunction for holes having a heavy effective mass and increases device resistance. As countermeasures against the problem, attempts to, for example, reduce resistance components in the heterojunction by introducing an AlGaAs semiconductor layer whose composition is gradually changed to the AlAs/GaAs heterojunction and doping only the AlAs side with p-type impurity have been made. However, it is very difficult to achieve device resistance of 50 xcexa9 or less in a device having an aperture of a small diameter realizing a single lateral mode.
A vertical cavity surface emitting laser having a structure of injecting a current not via an upper semiconductor multilayer reflecting mirror of high resistance is also being examined. As an example, FIG. 6 is a diagram showing the device structure of a vertical cavity surface emitting layer disclosed in Japanese Unexamined Patent Application No. Hei-11-204875, developed by the inventors herein. Shown are an n-electrode 601, an n-GaAs substrate 602, a lower multilayer reflecting mirror 603, a first GaAs spacer layer 604, a non-doped GaInNAs active layer 605, a second GaAs spacer layer 606, a current blocking layer 607, a p-current feeding layer 608, a third p-GaAS spacer layer 609, a p-electrode 610, and an upper multilayer reflecting mirror 611.
A current injected from the p-electrode 610 passes through the third spacer layer 609 and the current feeding layer 608, is led to an aperture defined by the current blocking layer 607, and fed to the active layer 605. That is, the current does not pass through the upper multilayer reflecting mirror 611, the device resistance is reduced. Further, in the structure, the current feeding layer 608 of which doping concentration is increased to p=1xc3x971020 cmxe2x88x923 is introduced, thereby realizing reduction in resistance components between the electrode and the aperture. Therefore, in the structure, device resistance of 50 xcexa9 or less can be achieved in a device having a small-diameter aperture realizing the single lateral mode.
However, when a number of lots of vertical cavity surface emitting lasers shown in FIG. 6 were manufactured in practice, although devices having very excellent characteristics in which a resistance value is about 20 xcexa9 were obtained, a problem such that reproducibility of characteristics among lots is low occurred. Particularly, devices having abnormally high resistance value and having bad electric characteristics were also manufactured. The cause was tracked down by the inventors herein and determined that something is wrong with the junction face between the third GaAs spacer layer 609 and the second GaAs spacer layer 606 as a part of the aperture. The junction face is a regrown interface formed by selectively etching the current blocking layer 607 and the current feeding layer 608 and regrowing the third spacer layer 609. Due to a malfunction in a regrowing process, the characteristics deteriorate.
Concretely, there is a case that reproducibility of etching of a small amount performed before regrowing is low, and crystallization of the interface is insufficient. On the other hand, if the small-amount etching process is omitted, Si is adhered to the interface for some reason in a process of selectively etching the current blocking layer 607 and current introducing layer 608, and the conduction type of the interface becomes an n-type. Even if the p-type third spacer layer 609 is regrown later, a p-n junction and a depletion layer caused by the p-n junction are formed and become a large resistance component.
A first object of the invention is to realize a semiconductor laser in which an influence of an impurity deposited on a regrown interface between electrodes of a semiconductor device is reduced.
A second object of the invention is to provide a high-speed and high-performance vertical cavity surface emitting laser having therein a regrown interface between electrodes, with reduced resistance between the electrodes without variations in characteristics when a number of lots are formed, and economical (low-cost) optical module and optical communication system with a simple configuration each using the vertical cavity surface emitting laser.
To achieve the objects, in a semiconductor laser of the invention having a plurality of semiconductor layers formed in a regrowing process between electrodes, a regrown interface or a face very close to the regrown interface is formed by a thin film containing dopants of high concentration.
To achieve the objects, a vertical cavity surface emitting semiconductor laser of the invention for emitting light perpendicular to substrate crystal has, on the substrate crystal, an active layer for generating light, a cavity structure in which the active layer is sandwiched reflecting mirrors to obtain a laser beam from the light generated from the active layer, and a regrown semiconductor layer provided between the active layer and one of the reflecting mirrors, and a regrown interface or a face very close to the regrown interface is formed by a thin film containing dopants of high concentration.
As the dopant, a dopant having a low diffusion constant is suitable and carbon is optimum. The delta doping performed to a position within 10 nm from the interface is substantially the same as the delta doping performed on the regrown interface since carriers move to the interface by a tunnel effect. The thickness of a layer to be delta-doped is 10 nm or less in consideration of a fabricating error.
According to the invention, in the semiconductor laser in which the regrown interface or a face very close to the regrown interface contains dopants of high concentration, by performing delta doping with a p-type dopant, the adverse influence of Si as an n-type dopant described by referring to FIG. 6 is lessened, and a resistance component of a p-n junction and a depletion layer caused by the p-n junction can be canceled. Specifically, a substance which is adhered on the interface and contaminates the interface is Si. On the contrary, in the case where a substance as a p-type dopant is adhered, by delta doping with an n-type dopant, the adverse influence of the p-type dopant can be canceled. The regrowing process is widely used also for semiconductor devices other than the vertical cavity surface emitting laser. The delta doping performed on the regrown interface to electrically lessen the contaminant deposit is effective to improve the interface and the device characteristics.
In a preferred embodiment of the vertical cavity surface emitting laser of the invention, in the vertical cavity surface emitting laser, the regrown interface is formed in a position close to the position of a node of a standing wave (preferably, in a position close to the position of the node within a distance of a xe2x85x9 wavelength or less).
The positions of the anti-node and node of a standing wave are uniformly determined by the distance from a reflecting mirror and an oscillation wavelength considering a refractive index of a substance in the cavity. The distance between the cavity components is accurately an integer times of the xc2xd wavelength thickness, and the anti-node exists every xc2xd wavelength thickness. Generally, the active layer is placed in the position of the anti-node of the standing wave in order to obtain the maximum gain (however, the position of the active layer does not determine the position of the anti-node of the standing wave). In this case, the nodes of the standing wave exists in positions of xc2xc, xc2xe, and {fraction (5/4)} wavelength thickness from the active layer. As shown in FIG. 7, when the position of the regrown interface is set in the node of the standing wave, since no light exists in the position of the node, the regrown interface does not become a factor of absorption and scattering. How a loss of light is influenced by the position of the interface was simulated. FIG. 8 shows the result of simulation of a reflection spectrum of an AlAs/GaAs multilayer reflecting mirror (25 cycles) having an interface subjected to C delta doping, serving as an optical absorber. The difference between 100% and reflectance indicates a loss of light. A loss in the case where the interface is in the position of the node is 0.04% which is the same as that in the case where there is no interface subjected to C delta doping.
On the other hand, in the case where the interface is in the position of the anti-node, a loss is 0.17% which is more than four times. Generally, in the vertical cavity surface emitting laser having a high density of light, increase of a loss largely exerts an influence on the optical characteristics of the device. Therefore, due to increase in a loss of four times, the efficiency does not simply deteriorate to xc2xc but oscillating operation itself may be checked.
Although the C delta doped interface is used as a factor of causing a light loss in the interface in the above simulation to simplify quantification, a similar result is obtained in the case of a regrown interface. When the interface is positioned, not necessarily in the accurate position of the node, within xc2x1xe2x85x9 wavelength thickness from a node, an effect at improving the characteristics of the vertical cavity surface emitting semiconductor laser is produced.
In the case where the thin film or the regrown interface containing the dopant is formed near the position of a node of a standing wave, an effect that both static characteristic and luminance characteristic of a semiconductor laser are improved is produced.